Automatic luminance and contrast adustment for display device

ABSTRACT

An apparatus and method is provided for dynamically modifying both the luminance and contrast of an image as it is displayed on a display unit in response to changing lighting conditions. Sensors are utilized to continually measure the luminance of the light illuminating the display unit and the display surround luminance. Measurement signals generated by the light sensors are processed to provide display luminance and contrast adjustment control signals that gradually cause the adjustment of the display unit&#39;s luminance and contrast in response thereto. Continual adjustment of the luminance and contrast of a display unit according to changing lighting conditions such that the brightness and contrast perception of the displayed image remains constant under the varying conditions. As a result, a number of advantages our obtained including: the brightness and contrast perception of images displayed on a display unit remains constant as the display device illumination changes; the brightness and contrast perception of images displayed on a display unit remains constant as the display unit surround luminance changes; and power consumption is decreased in dim environments.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method forautomatically adjusting luminance and contrast of a display unit.Specifically, the present invention is directed to an apparatus andmethod that adjust luminance and contrast as functions of ambient andsurround luminance.

BACKGROUND OF THE INVENTION

There are a variety of applications and devices in which a display unitis utilized to electronically display images. It is desirable to providea display unit for such applications and devices that is capable ofautomatically adjusting luminance and contrast without the need foroperator intervention. It is common, for example, to provide electroniccameras with image display units that must be utilized in a wide rangeof lighting conditions in which luminance levels vary greatly. Withoutsome form of automatic adjustment, the operator must constantly changesettings manually to compensate for changes in the lighting environment.

It has been well documented in the scientific literature that one'sbrightness and contrast perception of an image is significantly affectedby the ambient and surround luminance in which it is viewed. See, forexample, “Visibility of Transmissive Liquid Crystal Displays underDynamic Lighting Conditions”, J. H. Krantz, Human Factors, 1992, 34(5),615)-632, the contents of which are herein incorporated by reference.Brightness is defined as the attribute of a visual sensation accordingto which an area appears to emit more or less light.

Contrast is defined as the rate of change of the brightness of imageelements as a function of the relative luminance of the same imageelements in the original scene. Differences in these perceivedattributes can be related to the operator's state of visual adaptationto the ambient and surround luminance. For example, as the ambientluminance increases, the user begins to gradually adapt to the new lightlevel by: a) reducing the amount of light which enters the eye; and b)reducing the sensitivity of the light-sensitive elements of the eye. Thelight entering the eye from the display unit, however, is alsocorrespondingly reduced, hereby causing the display unit to appeardimmer to the operator. In response to his effect, the luminance of thedisplay unit needs to be increased to maintain the operator's perceptionof the displayed image. Conversely, as the ambient luminance decreases,the operator begins to gradually adapt by: a) increasing the amount oflight that enters the eye; and b) by increasing the sensitivity of thelight-sensitive elements of the eye. In this case, the light enteringthe eye from the display device unit is also increased, thereby causingthe display unit to appear brighter to the operator. The luminance ofthe display unit must then be decreased to maintain the user'sperception of the displayed image.

While the eye adapts to changes in ambient luminance, the process ofadaptation is not directly reversible. Instead, the human eye requires asignificant period of time to adapt to changes when ambient luminance isdecreased and a much shorter period to adapt to increases in ambientluminance. FIG. 1, for example, illustrates a plot of the darknessadaptation effect showing the amount of time it takes for an individualto visually adapt to changes in ambient luminance. As illustrated inFIG. 1, several minutes are required before the sensitivity of the eyereaches its full level of adaptation when the ambient luminance isdecreased. FIG. 2 is a plot of the lightness adaptation effect showingthe amount of time it takes for the eye to adapt to increased ambientluminance. As illustrated in FIG. 2, the eye's sensitivity changesdramatically over the first second after an increase in ambientluminance, continues to change significantly for a period of 20 to 30seconds, and then changes very slightly thereafter.

It has also been shown that the perceived contrast of an image changesas the luminance of the image surround changes. The surround of an imageis defined as the area around an image extending from its edge in alldirections. FIG. 3 is a plot which shows such an effect. The plotillustrates the relative brightness of image elements as a function oftheir relative luminances for an average, dim, and darkly illuminatedsurround, which shows that the perceived image contrast increases withincreasing surround luminance and decreases with decreasing surroundluminance.

It is also known that the physical luminance and contrast of the displayunit will be affected by lighting conditions. Assuming that the displayunit reflects light as if it were a perfectly Lambertian surface, it isknown that the luminance of the display unit in any ambient environmentcan be analytically determined from the equation:

La=Ld+Lr

where La represents the luminance of the display unit in current ambientlighting conditions, Ld represents the luminance of the display unit ina perfectly dark environment, and Lr represents the reflected luminanceof the display unit. If the amount of ambient illuminance is measured,Lr can be calculated from the equation:

Lr=I*R/π

where I is the ambient illuminance, R is the coefficient of reflectionfor the display device and π is the arithmetic constant PI. The contrastof the display device can be determined by calculating La for a darkpixel and La for a fully lit pixel on the display device and thencalculating the ratio of these parameters.

Contrast of Display=La^(fully lit pixel)/La^(dark pixel)

It should be noted that while R will usually be constant for mostdisplay units, this parameter may differ for dark and lit pixels forsome display units such as liquid crystal displays.

Still further, researchers have found that the optimum display luminanceis a function of display illumination level and this relationship cangenerally be explained by the equation:

 log L_(d)=a+b log(I)

where L_(d) is the display luminance, I is the illumination level, and aand b are constants fit to psychophysical data. See “The ABC's ofAutomatic Brightness Control”, R. Merrifield and L. D. Silverstein, SID88 Digest, 1988, pgs. 178-180, the contents of which are incorporatedherein by reference.

In view of the above, it is an object of the invention to provide anapparatus and method of dynamically modifying both the luminance andcontrast of an image as it is displayed on a display unit in response tochanging lighting conditions.

SUMMARY OF THE INVENTION

The invention provides an apparatus and method for dynamically modifyingboth the luminance and contrast of an image as it is displayed on adisplay unit in response to changing lighting conditions. Sensors areutilized to continually measure the luminance of the light illuminatingthe display unit and/or the display surround luminance. Measurementsignals generated by the light sensors are processed to provide displayluminance and contrast adjustment control signals that gradually causethe adjustment of the display unit's luminance and contrast in responsethereto.

More specifically, an apparatus for automatically controlling a displayluminance and contrast of a display device is provided that includes anillumination measuring mechanism for measuring a display illumination ofa display device; a surround luminance measuring mechanism for measuringa surround luminance of the display device; a calculating mechanism forcalculating a current display luminance and a current contrast settingfor the display device; a determining mechanism for determining a changein the display illumination and a change in the surround luminance basedon measurements performed by the illumination measuring mechanism andthe surround luminance measuring mechanism, respectively, a luminanceadjustment mechanism for calculating an updated display luminance whenthe determining mechanism determines a change in the displayillumination has occurred, and for adjusting the display luminance ofthe display device based on the updated display luminance to display animage, and a contrast adjustment mechanism for calculating an updatedcontrast setting when the determining mechanism determines a change inthe surround luminance has occurred, and for adjusting the contrast ofthe display device based on the updated contrast setting to display theimage.

In a preferred embodiment, the determining mechanism calculates thecurrent display luminance based on a determination of whether an initialdisplay illumination measured by the illumination measuring mechanism isdifferent from a default display illumination stored in memory.Similarly, the determining mechanism preferably calculates the currentcontrast setting based on a determination of whether an initial surroundluminance measured by the surround luminance measuring mechanism isdifferent from a default surround luminance stored in memory.

Still further, the luminance adjustment mechanism calculates the updateddisplay luminance based on the change in display illumination, andfurther performs a progressive time dependent adjustment of the displayluminance based on whether the updated display luminance is higher orlower than the current display luminance. The luminance adjustmentmechanism performs the progressive time dependent adjustment based on adegree of light adaptation when the updated luminance is higher than thecurrent display luminance, and performs the progressive time dependentadjustment based on a degree of dark adaptation when the updatedluminance is lower than the current display luminance.

The present invention provides continual adjustment of the luminance andcontrast of a display unit according to changing lighting conditionssuch that the brightness and contrast perception of the displayed imageremains constant under the varying conditions. As a result, a number ofadvantages are obtained including: the brightness and contrastperception of images displayed on a display unit remains constant as thedisplay device illumination changes; the brightness and contrastperception of images displayed on a display unit remains constant as thedisplay unit surround luminance changes; and power consumption isdecreased in dim environments. Other advantages and features of theinvention will become apparent from the following detailed descriptionof the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference tocertain preferred embodiments thereof and the accompanying drawings,where:

FIG. 1 is a plot of the adaptation response of the human visual systemwhich shows the amount of time it takes to visually adapt to decreasesin ambient luminance;

FIG. 2 is a plot of the adaptation response of the human visual systemwhich shows the amount of time it takes to visually adapt to increasesin ambient luminance;

FIG. 3 is a plot which shows how the relative brightness of imageelements vary as a function of their relative luminances for an average,dim, and darkly illuminated surround;

FIG. 4 is a block diagram of various elements of an electronic camerautilizing a display unit in accordance with the present invention; and

FIGS. 5 and 6 are flow diagrams illustrating the operation of theelectronic camera shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 is a block diagram in schematic form of an electronic camera 10utilizing a display device in accordance with the invention. The cameraincludes an area image sensor 12, a display illumination sensor 14, asurround luminance sensor 16, a microprocessor 18, a memory 20, and adisplay device 22. The area image sensor 12 generates image data inresponse to an imaged scene, as will be described in greater detail, andtransfers the image data to the microprocessor 18 for subsequent storagein memory 20 and display on the display unit 22. In a preferredembodiment of the invention, the display illumination sensor 14 isutilized to obtain a measurement of the display illuminance and thesurround luminance sensor 16 is utilized to obtain a measurement of thesurround luminance. The display illuminance and surround luminancemeasurements are supplied to the microprocessor 18 which determines theappropriate luminance and contrast of the display device 22. The processof display illuminance measurement, surround luminance measurement,image processing, and image display preferably continues as long aspower is supplied to the display device 22 by the power supply 24.

In operation, an image of a subject focused by a lens 26 passes throughan aperture plate 28 and a low pass filter 30, which can be made of acrystalline material or include a transport substrate with varioustransparent spots formed on the surface of the substrate. The area imagesensor 12, under the control of the microprocessor 18, delivers analogsignals to an analog signal processor 32. The analog signal processor 32applies its output to an analog-to-digital converter (A/D) 34 which isunder control of a timing generator 36. The digitized signals generatedby the A/D 34 are received by the microprocessor 18 and are placed inappropriate memory locations as stored pixels in the memory 20.Conventional imager drivers 38 that provide drive signals to the areaimage sensor 12 are also controlled by the timing generator 36. Inaddition, the timing generator 36 provides control signals to displayinterface electronics 40 and to a digital-to-analog converter (D/A) 42,as well as to the memory 20. The timing generator causes stored pixelsto be delivered to the digital-to-analog converter 42 which converts thepixels to an analog image signal that is supplied to the displayinterface electronics 40. Display interface electronics 40 provides itsoutput to a display controller 44 which, in turn, provides appropriateanalog information to the display device 22.

FIG. 5 illustrates in flow diagram form the processing provided by themicroprocessor 18 on a digital image sensor image already stored in thememory 20. It will be understood that the steps set forth below can beemployed on a computer readable storage medium. The computer readablestorage medium may comprise, for example, magnetic storage media such asmagnetic disc (such as a floppy disc) or magnetic tape; optical storagemedia such as optical disc, optical tape, or machine readable bar code;solid state electronic storage devices such as random access memory(RAM), or read only memory (ROM); or any other physical device or mediumemployed to store a computer program.

In accordance with an embodiment of the invention, a digital imageproduced by the camera of FIG. 4 is stored in memory 20. The first step(S1) is to turn the power on to both the camera 10 and the displaydevice 22. The image to be displayed is then retrieved (S2) from thememory 20 by the microprocessor 18. Next, the microprocessor 18retrieves default values (S3) from the memory 20 for the displayilluminance (I_(def)), the surround luminance (S_(def)), the displayluminance (L_(def)) and image contrast settings (λ_(def)). An initialdisplay illuminance reading (I_(I)) is then taken from the displayillumination sensor 14 (S4). A decision is made (S5) if the measuredinitial display illumination reading is different from its defaultvalue. If the answer is “yes”, then the current luminance of the displaydevice 22 is calculated based upon the display illumination reading (S6)using the equation:

log L_(d)=a+b log(I)

previously described above. The display device 22 will be adjusted toproduce a luminance value that is as close to the calculated value aspossible. Adjustment can be accomplished in a number of ways dependingon the display technology. In some devices, the value is obtained byadjusting the power input to each element in the display by adjustingits drive voltage and/or current supplied to each element. In a backlitdisplay, for example a backlit LCD display, the power to the backlightis adjusted. An initial reading is then taken from the surroundluminance sensor 16 (S7). A decision is made to determine if themeasured initial surround luminance reading is different from itsdefault value (S8). If the answer is “yes”, then the contrast of theimage on the display device 22 is adjusted based on the surroundluminance reading (S9) by calculating the luminance at each gray levelY(x) from the equation:

logY(x)=logL_(dmax)+1/λ(log((L(x)+16)/116))

where x represents each gray level, L(x) is the desired perceivedluminance of the display at each gray level, Y(x) is the physicalluminance of the display at each gray level, L_(dmax) is the peakluminance value of the display, and λ is a contrast parameter that isadjusted based on the surround luminance. The value of the contrastparameter is calculated from an equation in the following form:

λd=aS^(c)+b

where a, b and c are constants that are derived by fitting the datapoints obtained from any number of psychophysical experiments and S isthe luminance detected from the surround luminance sensor. The image isthen displayed on the display device 22 using the adjusted currentvalues for the display device luminance and contrast setting (S10).

From this point on, the display luminance and contrast setting of thedisplay device 22 are continually updated in response to changes in thereadings from the display illumination sensor 14 and the surroundluminance sensor 16 in a looping fashion as defined by the balance ofthe flow diagram. Namely, as shown in FIG. 6, another reading from thedisplay illumination sensor 14 is taken (S11) and a decision is made asto whether the reading is different from the previous reading (S12). Ifthe answer is yes, then the amount of difference from the previousdisplay luminance is calculated (S13) and a determination is made as towhether the new value is higher or lower than the previous value (S14).A progressive time dependent adjustment to the display luminance is thenmade in relation to whether a degree of light adaptation or darkadaptation is required (S115, S16), as an observer will adapt to changesin the environment as the ambient environment changes. Through theadaptation process, the observer becomes gradually more sensitive tolight when adapting to darker environments and gradually less sensitiveto light when adapting to brighter environments. To determine the timedependent adjustments, the change in display illumination is detected,the change in luminance that is required for complete adaptation iscalculated, and the time dependent adjustments to the display luminanceare determined from the curves describing the rate of lightness anddarkness adaptation (See FIGS. 1 and 2) and progressively applied to thedisplay unit the complete adaptation luminance value is reached. Afurther reading is then taken (S17) from the surround luminance sensor16 and a decision is made as to whether the reading is different fromits previous reading (S18). If the answer is yes, then image contrast iscalculated based upon surround luminance (S19) and an adjustment orupdating of image contrast is made (S20). This process of updating thedisplay luminance and contrast of the image on the display device 22 iscontinued until power is turned off.

The invention has been described with reference to certain preferredembodiments thereof. It will be understood, however, that modificationsand variations are possible within the scope of the appended claims. Forexample, while the illustrated embodiment shows the use of a separatedisplay illumination sensor and a surround luminance sensor to measureillumination and surround luminance, an equivalent means for performingthis function would be the use of a single sensor that directly measureseither illumination or surround luminance, with the other value beingestimated based on the direct measurement. Namely, one directmeasurement can be made with one sensor and the microprocessor thencalculates the other value from the direct measurement, based on theassumption that the surround area has a certain amount of reflectance.If the display illuminance measurement is taken directly, the surroundluminance value can be calculated from that measurement and an assumedsurround reflectance. Similarly, if a luminance measurement is directlytaken, the display illumination can be calculated from the measurementand an assumed surround reflectance. Other variations are also possible,including adjusting contrast in a progressive manner similar to theadjustment of luminance. The invention is applicable to different typesof display devices and may be readily employed in a variety of devicesthat utilize electronic imaging.

PARTS LIST

10 digital camera

12 area image sensor

14 display illumination sensor

16 surround luminance sensor

18 microprocessor

20 memory

22 display unit

24 power supply

26 lens

28 aperture plate

30 low pass filter

32 analog signal processor

34 analog-to-digital converter (A/D)

36 timing generator

38 imager drivers

40 display interface electronics

42 digital-to-analog converter (D/A)

44 display controller

What is claimed is:
 1. An apparatus for automatically controlling adisplay luminance and contrast of a display device, the apparatuscomprising: a) an illumination measuring mechanism for measuring adisplay illumination of a display device; b) a surround luminancemeasuring mechanism for measuring a surround luminance of the displaydevice; c) a calculating mechanism for calculating a current displayluminance and a current contrast setting for the display device; d) adetermining mechanism for determining a change in the displayillumination and a change in the surround luminance based onmeasurements performed by the illumination measuring mechanism and thesurround luminance measuring mechanism, respectively; e) a luminanceadjustment mechanism for calculating an updated display luminance whenthe determining mechanism determines that a change in the displayillumination has occurred, and for adjusting the display luminance ofthe display device based on the updated display luminance to display animage; and f) a contrast adjustment mechanism for calculating an updatedimage contrast when the determining mechanism determines that a changein the surround luminance has occurred, and for adjusting the imagecontrast on the display device based on the updated contrast setting todisplay the image; wherein: the current display luminance is replacedwith the updated display luminance and the current contrast setting isreplaced with the updated contrast setting; the luminance adjustmentmechanism calculates the updated display luminance based on the changein display illumination, and further performs a progressive timedependent adjustment of the display luminance based on whether theupdated display luminance is higher or lower than the current displayluminance; the luminance adjustment mechanism performs the progressivetime dependent adjustment based on a degree of light adaptation when theupdated luminance is higher than the current display luminance; and theluminance adjustment mechanism performs the progressive time dependentadjustment based on a degree of dark adaptation when the updatedluminance is lower than the current display luminance.
 2. An apparatuscomprising: a) imaging means for generating electronic image datacorresponding to an imaged scene; b) a display means for displaying theelectronic image data generated by the imaging means; c) illuminationmeasuring means for measuring a display illumination of a displaydevice; d) surround luminance measuring means for measuring a surroundluminance of the display device; and e) control means for controlling adisplay luminance and contrast of the display means, wherein saidcontrol means includes: i) calculating means for calculating a currentdisplay luminance and a current contrast setting for the display device;ii) determining means for determining a change in the displayillumination and a change in the surround luminance based onmeasurements performed by the illumination measuring means and thesurround luminance measuring means, respectively; iii) luminanceadjustment means for calculating an updated display luminance when thedetermining means determines that a change in the display illuminationhas occurred, and for adjusting the display luminance of the displaydevice based on the updated display luminance to display an image; andiv) contrast adjustment means for calculating an updated contrastsetting when the determining means determines that a change in thesurround luminance has occurred, and for adjusting the contrast of thedisplay device based on the updated contrast setting to display theimage; wherein: the current display luminance is replaced with theupdated display luminance and the current contrast setting is replacedwith the updated contrast setting; the luminance adjustment meanscalculates the updated display luminance based on the change in displayillumination, and further performs a progressive time dependentadjustment of the display luminance based on whether the updated displayluminance is higher or lower than the current display luminance;luminance adjustment means performs the progressive time dependentadjustment based on a degree of light adaptation when the updatedluminance is higher than the current display luminance; and theluminance adjustment means performs the progressive time dependentadjustment based on a degree of dark adaptation when the updatedluminance is lower than the current display luminance.
 3. A method ofcontrolling a display luminance and contrast of a display devicecomprising: a) measuring a display illumination of a display device; b)measuring a surround luminance of the display device; c) calculating acurrent display luminance and a current contrast setting for the displaydevice; d) determining a change in the display illumination and a changein the surround luminance; e) calculating an updated display luminancewhen a change in the display illumination has occurred; f) adjusting thedisplay luminance of the display device based on the updated displayluminance to display an image; g) calculating an updated contrastsetting when a change in the surround luminance has occurred; h)adjusting the contrast of the display device based on the updatedcontrast setting to display the image; and i) replacing the currentdisplay luminance with the updated display luminance and the currentcontrast setting with the updated contrast setting; wherein: the updateddisplay luminance is calculated based on a change in displayillumination, and a progressive time dependent adjustment of the displayluminance is performed based on whether the updated display luminance ishigher or lower than the current display luminance; the progressive timedependent adjustment is performed based on a degree of light adaptationwhen the updated luminance is higher than the current display luminance;and the progressive time dependent adjustment is performed based on adegree of dark adaptation when the updated luminance is lower than thecurrent display luminance.
 4. An apparatus for automatically controllinga display luminance comprising: a) means for measuring displayillumination; b) calculating means for calculating a current displayluminance based on the measured display illumination; c) determiningmeans for determining a change in the display illumination based onmeasurements performed by the means for measuring display illumination;and d) luminance adjustment means for calculating an updated displayluminance when the determining means determines a change in the displayillumination has occurred; wherein the luminance adjustment meanscalculates the updated display luminance based on the change in displayillumination and performs a progressive time dependent adjustment of thedisplay luminance based on whether the updated display luminance ishigher or lower than the current display luminance; wherein theluminance adjustment means performs the progressive time dependentadjustment based on a degree of light adaptation at a first rate whenthe updated luminance is higher than the current display luminance; andwherein the luminance adjustment means performs the progressive timedependent adjustment based on a degree of dark adaptation at a secondrate which is slower than the first rate when the updated luminance islower than the current display luminance.